The present invention relates to motor vehicle seat belts, and its subject is, particularly, a self-locking strap guide for a seat belt, intended in particular for a land motor vehicle.
As is known, the authorities have enacted regulations which stipulate that vehicles must be equipped with seats provided with seat belts for their occupants. This applies, in particular, to land motor vehicles.
The majority of modern land motor vehicles are provided with a seat belt having three anchorage points and a retractor. Consequently, when such a belt is unreeled and worn by the occupant of a seat, it essentially comprises two working strap portions: a chest portion and a lap portion which remain applied against the occupant by virtue of the tension generated by the retractor. The chest portion extends diagonally, in the manner of a shoulder strap, across the chest of the occupant, whilst the lap portion extends transversely, near waist level. The retractor, in addition to the tension which it exerts in order to hold the strap in contact with its wearer, also makes it possible, when the belt is not being used, to return it to the retracted position so that it does not lie untidily on the seat or on the floor of the vehicle. The retractor is usually provided with an acceleration/deceleration detector which locks it in such a way that the strap cannot be unreeled when it is stressed suddenly by its occupant so that the latter is thus held back, when necessary, in his seat. The length of strap unreeled for a seat belt worn by an occupant of medium build is of the order of two meters or more if there is added to these working portions the operating portion which extends between the retractor, often placed near the floor, and the strap guide situated at shoulder height of a seated occupant.
When a vehicle is subjected to an abrupt deceleration, such as for example that which occurs in the event of a violent impact of longitudinal component, all the kinetic energy of the occupant is absorbed by the seat belt so as to hold him back in his seat and prevent him from being thrown forwards. This kinetic energy absorbed by the seat belt is translated into substantial forces exerted, in particular, on the retractor and on the whole unreeled length of strap, which thus tends to lengthen.
It can thus be understood that, in order to meet the standards currently in force, a retractor must be constructed, and a strap manufactured which are particularly strong and hence expensive.
One of the solutions which has already been proposed consists in providing for only part of the unreeled length of strap, and not all of it, to be used in order to absorb the kinetic energy of the occupant. By not transmitting the forces to the reactor via the operating portion, it is then possible to construct the former less robustly using materials which are lighter and less expensive than the traditional materials.
One of the techniques proposed for the implementation of such a solution consists in using a strap guide, which is situated at the upper extremity of the chest portion, with a catch mechanism. In the event of an abrupt deceleration, this catch mechanism jams the strap in the guide and prevents the forces, absorbed solely by the chest portion and the lap portion, from being transmitted to the remainder of the straps or to the retractor. Embodiments of such a technique are, for example, disclosed in the documents WO 86/00 585 and EP 0 271 183. The proposals described in these documents make use of catch mechanisms which employ wedge-locking mechanisms. These mechanisms include numerous and complex components and are consequently expensive to produce, assemble and mount, and their reliability is also dubious.